For an initial synchronization in a cellular system, a terminal should be able to realize time synchronization and frequency synchronization based on signals of a base station. In addition, the terminal should be able to perform a cell search. After realizing the initial synchronization, the terminal should be able to trace time and frequency, and also, for a handover, it should be able to realize time and frequency synchronizations and perform a cell search with respect to an adjacent cell.
Exemplary techniques for a cell search are found at a thesis published in IEEE VTC Fall, OFDM Section IV-6 (September 2005), in which two schemes of frame structures are proposed. In a first scheme, one frame is divided into four time blocks, where synchronization identification information, cell group identification information, unique cell identification information, and synchronization identification information are allocated in the four time blocks, respectively. In a second scheme, one frame is divided into four time blocks, where synchronization identification information and unique cell identification information are allocated in first and third time blocks, and synchronization identification information and cell group identification information are allocated in second and fourth time blocks.
According to the first scheme, the symbol synchronization is obtained only in the first time block. Therefore, a rapid synchronization acquisition within a predefined 4.5 ms may become impossible during turning on of the terminal or a handover between heterogeneous networks. In addition, it is difficult to acquire a diversity gain by accumulating synchronization identification information for a rapid synchronization acquisition.
On the other hand, according to the second scheme, for frame synchronization, the unique cell identification information or the cell group identification information should be correlated simultaneously with the synchronization acquisition. Therefore, the cell search process is complex and a rapid cell search is difficult.
As another technique for the cell search, it has been proposed to use a separate preamble in order to acquire synchronization and search a cell. However, such a technique cannot be applied to a system that does not involve a preamble. In addition, since a preamble is located forward in a frame, the terminal should wait for a subsequent frame in order to acquire synchronization at a time that is not a start of the frame. In particular, although the terminal should acquire the initial symbol synchronization within 4.5 msec when performing hand-off among a GSM mode, a WCDMA mode, and a 3GPP LTE mode hand-off, there may be an occasion in which the initial symbol synchronization cannot be acquired within the 4.5 msec since the synchronization is only acquired by a frame unit.